System and Method for Advanced Adaptive Pseudowire

ABSTRACT

A system and method for separating clock recovery for a pseudowire communication. An incoming signal is received for a pseudowire communication. The incoming signal is separated into a first signal and a second signal. Packets within the first signal are ordered in a first register. A clock signal is extracted from the second signal in a second register to generate a modified clock signal. A delay is incurred during generating of the modified clock signal. The first signal is communicated utilizing the modified clock signal.

BACKGROUND

The use of and development of communications has grown nearlyexponentially in recent years. The growth is fueled by larger networkswith more reliable protocols and better communications hardwareavailable to service providers and consumers. One embodiment of animportant service is pseudowire. A pseudowire is an emulation of anative service over a packet switched network (PSN). The native servicemay be low-rate or high-rate time domain multiplexing (TDM), synchronousoptical network (SONET)/SDH, asynchronous transfer mode (ATM), framerelay, or other similar service while the PSN may be Ethernet,multi-protocol label switching (MPLS), or other internet protocol (IP).

One critical issue in implementing TDM pseudowire is clock recovery. Innative TDM networks, the physical layer carries highly accurate timinginformation along with the TDM data. However, when emulating TDM overPSNs, this physical layer clock may be absent. TDM timing standards maybe exacting and conformance with such standards may require innovativemechanisms to adaptively reproduce the TDM timing or original clocksignal.

SUMMARY

One embodiment includes a system and method for separating clockrecovery for a pseudowire communication. An incoming signal may bereceived for a pseudowire communication. The incoming signal may beseparated into a first signal and a second signal. Packets within thefirst signal are ordered in a first register. A clock signal may beextracted from the second signal in a second register to generate amodified clock signal. A delay may be incurred during generating of themodified clock signal. The first signal may be communicated utilizingthe modified clock signal.

Another embodiment includes a system for clock recovery. The system mayinclude a separator operable to separate an incoming signal into a firstsignal and a second signal. The system may also include a first registerin communication with the separator. The first register may be operableto manage jitter and reorder packets for the first signal. The systemmay also include a second register in communication with the separator.The second register may be operable to recover a modified clock signal.The second register may introduce a second delay to recover the modifiedclock signal. The system may also include a transmitter in communicationwith the first register and the second register. The transmitter may beoperable to play out the first signal synchronously utilizing themodified clock signal.

Yet another embodiment includes a clock recovery unit. The clock recoverunit may include a processor for executing a set of instructions and amemory for storing the set of instructions. The set of instructions maybe operable to receive an incoming signal, separate the incoming signalinto a first signal and a second signal, ordering packets for the firstsignal in a first register, recover a modified clock signal from thesecond signal in a second register, ticks of the modified clock signalcorresponding to ticks of an original clock signal with a delay forrecovering the modified clock signal, and play out the first signal inresponse to the modified clock signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention are described indetail below with reference to the attached drawing figures, which areincorporated by reference herein and wherein:

FIG. 1 is a pictorial representation of a communications environmentimplementing pseudowire in accordance with an illustrative embodiment;

FIG. 2 is a block diagram of a clock recovery system in accordance withan illustrative embodiment; and

FIG. 3 is a flowchart of a process for recovering a clock in accordancewith an illustrative embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

An illustrative embodiment provides a system and method for adaptiveclock recovery for pseudowire applications. In one embodiment, the datastream processing which may include ordering bits and/or packets andcompensating for jitter may be implemented in a first small register orbuffer with minimal delay. Recovery of a clock signal may be performedin parallel by a second larger register or buffer with a greater delay.The first signal may be played out for synchronous communicationsutilizing the recovered clock signal. The ticks of the original clocksignal correspond to the ticks of the recovered clock signal even thoughthere is a delay to generate the recovered clock signal. The ticks ofthe clock are time shifted, but identical for purposes of furthercommunicating synchronous communications. The original clock signal maybe a recovered signal or received directly from a reference clock. Theillustrative embodiments may be utilized for any connections, systems,equipment, devices, or parties performing adaptive clock recovery.

FIG. 1 is a pictorial representation of a communications environmentimplementing pseudowire in accordance with an illustrative embodiment.The communications environment 100 may include any number of systems,equipments, devices, operators, nodes, and other elements. In oneembodiment, the communications environment 100 may include an Ethernetnetwork 105, a communications device 110, a clock source 115, usernetwork 120, customer premise equipment (CPE) device 125, and anexchange 130.

The different elements and components of the communications environment100 may communicate using hardwired connections, such as fiber optics,T1, cable, DSL, high speed trunks, and telephone lines. Alternatively,portions of the communications environment 100 may include wirelesscommunications, including satellite connections, time division multipleaccess (TDMA), code division multiple access (CDMA), global systems formobile (GSM) communications, personal communications systems (PCS),WLAN, WiMAX, or other frequently used cellular and data communicationsprotocols and standards.

The Ethernet network 105 is a packet-switched network that allowsendpoints and intermediary devices to communicate. A packet is aformatted unit of data carried by a packet node or data network. Asshown, the CPE device 125 may receive a signal from the communicationsdevice through the Ethernet network 105. The CPE device may obtain orrecover the line clocking from an external circuit or from an embeddedor encoded clock based on the clock source 115. The clock source 115 isone embodiment of a highly accurate reference clock that may be utilizedto maintain a universal time constant or other time reference. The clocksource 115 may be a Stratum, atomic, GPS, radio frequency clock, orother time keeping device suitable for maintaining highly accurate timeinformation. The clock source 115 may provide the reference clock tosecondary nodes, such as the CPE device 125 and exchange 130. In oneembodiment, the clock source 115 may represent the line or internalclocking recovered by intermediary devices that is further distributedto devices, such as the CPE device 125. As a result, the clock signalmay retrieved directly from the clock source 115 or indirectly based onpreviously recovered clock signals and then further distributed toadditional clocks, nodes, devices, and systems. The hierarchy of timesynchronization is essential for the proper functioning of thecommunications environment 100 as a whole.

In one embodiment, the Ethernet network 105 may include an Ethernettopology, such as an Ethernet ring. For example, the Ethernet ring maybe a metro Ethernet network. A metro Ethernet is a layer 2 network thatis based on the Ethernet standard and that covers a metropolitan area.Metro Ethernet may be utilized as a metropolitan access network toconnect subscribers and businesses to a wide area network (WAN), such asthe Internet. In other embodiments, the Ethernet network 105 may be anynumber of data networks suitable for data or packet communications.

One or more large businesses, organizations, or other groups may alsouse metro Ethernet to connect branch offices to an intranet. TheEthernet network 105 may be used because Ethernet supports highbandwidth connections and may be easily integrated with and connected tocorporate and residential customer devices, networks, or otherresources. In one embodiment, the Ethernet ring may provide a nextgeneration replacement of SONET rings.

In one embodiment, the Ethernet network 105 includes a number ofswitches interconnected through fiber optic connections. The Ethernetnetwork 105 may also be any topology, structure, or design suitable forcommunication, such as hub-and-spoke, star, point-to-point, full mesh,partial mesh, or other Ethernet architectures. The Ethernet network 105may include any number of devices, elements, and connections. Forexample, in order to communicate data through the fiber optics, anynumber of routers, splices, amplifiers, media converters, modulators,multiplexers, light generators, and other elements may be used.

The Ethernet network 105 may utilize a hierarchy, including corefunctionality, distribution functionality, and access functionality orlayers. For example, the core may be the backbone of the communicationsnetwork for communicating signals, data, and information from one pointto another. The switches, multiplexers, routers, or other data elementsof the Ethernet network are central points of connections for computersand other communications equipment in a communications network. Thecommunications devices of the Ethernet network 105 may be located at oneor more central offices, nodes, multi-tenant buildings, hardenedcabinets, or other service provider or customer facilities.

Switches and other devices may need to communicate a clock signalthrough the Ethernet network 105 to the CPE device 125 or an edgeaggregator. The CPE device 125 is the point, connection, or device atwhich access legs in communication with customer or user equipmentinterconnect with the Ethernet network 105. Clock timing may beparticularly important for TDM emulation over packet communications, orother circuit based or time slot based communications protocols andstandards over packet communications. Synchronous TDM includes T1,SONET/SDH, and integrated services digital network (ISDN). Additionally,a clock signal may be required to perform TDM over a non-synchronouspacket network with such emulation modes as a differential modepseudowire as previously described.

In one embodiment, the user network 120 is a TDM network that mayrequire a clock signal, such as that originating from the clock source115. The illustrative embodiments may recover a clock signal for furthercommunications from the CPE device 125 utilizing real time clockextraction or by disciplining a local clock at the CPE device 125 forthe same purpose. The user network 120 and Ethernet network 105 maycommunicate with any number of networks which may include wirelessnetworks, data, or packet networks, cable networks, satellite networks,private networks, publicly switched telephone networks (PSTN), or othertypes of communication networks. The networks of the communicationsenvironment 100 may represent a single communication service provider ormultiple communications services providers. The features, services, andprocesses of the illustrative embodiments may be implemented by the CPEdevice 125. In another embodiment, the system and method hereindescribed may be performed by one or more devices of the communicationsenvironment 100 independently, or as a networked implementation.

The CPE device 125 may be a communication device using time domainmultiplexing to provide telecommunications services to a particularsubscriber or group of subscribers. The CPE device 125 may be amultiplexer, media converter or other similar device. The CPE device 125may communicate with the exchange 130. The exchange 130 may be locatedat an organization location serving as a private branch exchange or at alocal telephone company's central office, or other similar location. Theexchange 130 may be a local exchange, a wire-line switch, or a publicexchange. The exchange 130 may provide dial-tone, calling features, andadditional digital and data services to subscribers, such as a networkof telephones utilized by a user.

The user network 120 is an example of a user, customer, network, system,equipment, device, residence, building, or location of a person or groupthat may utilize any number of communications services. The user network120 may include any number of individual or networks of devices forvoice communication and data communication including, but not limited totelephones (i.e., voice over Internet protocol (VoIP), plain oldtelephone service (POTS), servers, clients, and other communicationsdevices.

Communications networks may constrain end-to-end delays utilizing anynumber of standards (i.e., ITU-T G.114/G.131). Circuit emulationservices may use packet buffering at the far end of a packet flow tosmooth jitter, reorder packets, and recover the clock signal for playingout synchronous circuits. The buffering size and resulting latency addedby TDM pseudowire depends on the circuit emulation packetization rateand resulting number of packets and packet size, but typically the sizeof a single frame may range from one to multiple milliseconds of bufferspace. The CPE device 125 may retrieve the clock signal from a signalreceived from the communications device 110 to perform synchronouscommunications with the user networks 120 and other TDM networks. Asfurther described herein, the clock signal may be determined by:duplicating or separating the incoming signal for data path processingin a first shorter buffer and clock recovery in a separate longerbuffer; adding a delay to the clock signal; and then recovering a phaseor time shifted clock signal for updating or disciplining a local clockof the CPE device 125 for user with pseudowire or other synchronizedcommunications.

One embodiment may remove the clock recovery function from the data pathjitter buffer so that the jibber buffer may be shortened. Adding a phaseor time shifted clock recovery buffer of suitable depth also providesgreater stability and accuracy to the clock recovery function whichdepends upon statistical measurements made off a group of received bitsor frames. As a result, the parallel clock recovery buffer does notintroduce additional delay into the circuit emulation packet path.Therefore, the clock recovery buffer size may be very large (formathematical stability) improving clocking and stability withoutintroducing path delay for the bearer traffic.

FIG. 2 is a block diagram of a clock recovery system in accordance withan illustrative embodiment. In one embodiment, a CPE or othercommunications device may include a clock recovery system 200. Forexample, the clock recovery system 200 may integrated with a receiver ortransceiver of the CPE device. The clock recovery system 200 may be anapplication specific integrated circuit (ASIC), field programmable gatearray (FPGA), instructions stored in memory, individual circuits, logicelements, modules, chips, or any number of other elements. In oneembodiment, the clock recovery system 200 may be integrated with the CPEdevice 125 of FIG. 1.

The clock recovery system 200 may include any number of elementsincluding, but not limited to, an incoming signal 205, a separator 210,a data path register 215, a clock recovery register 220, a delayincrementer 225, a clock 230, and a transmitter 235. In anotherembodiment, the clock recovery system may be a separate device which mayinclude a processor, memory, other components which may include busses,motherboards, circuits, ports, interfaces, cards, converters, adapters,connections, transceivers, displays, antennas, and other similarcomponents. The processor is circuitry or logic enabled to controlexecution of a set of instructions. The processor may be amicroprocessor, digital signal processor, application-specificintegrated circuit (ASIC), central processing unit, or other devicesuitable for controlling an electronic device including one or morehardware and software elements, executing software, instructions,programs, and applications, converting and processing signals andinformation, and performing other related tasks. The processor may be asingle chip or integrated with other computing or communicationselements.

The memory is a hardware element, device, or recording media configuredto store data for subsequent retrieval or access at a later time. Thememory may be static or dynamic memory. The memory may include a harddisk, random access memory, cache, removable media drive, mass storage,or configuration suitable as storage for data, instructions, andinformation. In one embodiment, the memory and processor may beintegrated. The memory may use any type of volatile or non-volatilestorage techniques and mediums.

Packets received at the clock recovery system 200 may arrive with adelay that has a random component, known as packet delay variation. Therandomness of the delay may be accounted for by the data path register215 by reading out data at a constant rate for delivery to TDM end-userequipment, such as an exchange. The precise rate at which the data is tobe clocked out of the data path register 215 may need to be determinedaccording to a source time reference.

The clock 230 is an efficient timing device, but the clock 230 still mayrequire disciplining based on a higher accuracy clock. To ensure qualitycommunications, a high order of synchronization may be required,therefore the clock 230 needs to be synchronized against the referenceclock frequently. In one embodiment, the synchronization may beperformed for each tick of a clock. In one embodiment, the tick of theclock may correspond to the leading or trailing edge of a datatransition. The clock ticks may be for picoseconds, nanoseconds,microseconds, milliseconds, decaseconds, or other time intervals. In oneembodiment, the clock recovery system 200 may not include the clock 230,but rather the data stream processed by the data path register 215 maybe played out or communicated directly utilizing the clock signalrecovered by the clock recovery register 220.

The separator 210 is a device or module operable to separate, replicate,or duplicate the incoming signal 205 exactly. The separator 210 convertsthe incoming signal 205 into at least two data streams communicated toat least the data path register 215 and the clock recovery register 220.For example, the separator 210 may use a multi-cast function to splitthe incoming signal 205 into two signals. In another example, theseparator 210 may act as mirror. In one embodiment, the separator is apacket mirror operable to exactly copy an original incoming data signalinto two signals. In another embodiment, the separator 210 is a packetsniffer or packet analyzer operable to duplicate the incoming signal foranalysis by the data path register 215 and clock recovery register 220.

The data path register 215 is a data register implemented using digitalelectronics and other suitable hardware or software. In one embodiment,the data path register 215 may include a jitter buffer. The jitterbuffer is an electronic queue operable to temporarily store data packetsso that a continuous playout of the packets of the incoming signal maybe ensured. The data path register 215 may also be configured to reorderpackets that arrive out of order and playout the packets from the datapath register 215 for further communication by the transmitter 235.Continuous playout ensures a consistent rate of communication and thecontinuity of the packets within the signal. Continuity of packets isespecially important for voice communications in which missing or out oforder packets may impair the quality of voice communications. Mobilecommunications system that use more than one voice path between themobile switch and the customer handset require similar path delayproperties to facilitate the users movement from on tower and subsequentbearer path to another. When circuit emulation is used next to a legacyT1 system, a large buffer may induce so much differential path delaythat callers traversing from one path to another during a “hand-off”exceed the systems capabilities to successfully transition which mayresult in dropped calls. The data path register 215 or other elements ofthe clock recovery system 200 may further process the data or packets toensure communications are perpetuated without delays in data, timing, orcommunications characteristics.

By separating the incoming signal into two separate parallel buffers orregisters, the size of the data path register 215 may be reduced toprevent delays and dropped calls. For example, in some cases calls maybe dropped if the delay caused in part by the data path register 215exceeds 16-20 milliseconds. In some existing buffers, retrieving theclock may add an additional 8 milliseconds to the delay. Such a delaymay make the data stream unusable for tower backhaul which typicallyrequires a delay of less than 5-8 milliseconds. For example, when auser's cellular handset switches primary communications between towerswith differential delays for the voice streams a delay beyond 8 ms maycause an ongoing call to be dropped. In one embodiment, the data pathregister 215 may be required to buffer a certain amount of packets inorder to play out the packets at proper intervals.

The clock recovery register 220 is a data register implemented usingdigital electronics or any other suitable hardware or software. Theclock recovery register 220 is operable to recover the clock signal fromthe data stream. In one embodiment, the clock recovery register 220utilizes transitions, such as the leading or trailing edge of bits orother signal characteristics to determine or extract the reference clocksignal. For example, a clock signal may be extracted and phase alignedwith the incoming data stream utilizing a phase locked loop (PLL) Anynumber of encoding schemes may be utilized as part of the incomingsignal 205.

The size, length, or time to extract or determine the clock signal fromthe data stream may vary. The clock signal may not be recoveredutilizing a direct or real time correlation to the bits or elements ofthe data signal utilized by the data path register 215. For example, thedata path register 215 may only require 5 ms to process the data foradditional transmissions when the clock recovery register may require 30ms to extract an accurate clock signal. As a result, the clock signalmay be utilized by the transmitter 235 to play out the data utilizingthe delayed signal. Similarly, the clock 230 may be disciplinedutilizing clock ticks even though the delay associated with the clockrecovery register may be significant when communications are firstreceived by the clock recovery system 200.

In another embodiment, the clock recovery register 220 may include anumber of different clock recovery registers. For example, the clockrecovery register 220 may include 15 ms, 30 ms, and 100 ms registers. Asa result, the clock recovery register 220 may be able to recover a clocksignal even in the event of a temporary fail over of data interruptionbecause the clock recovery register 220 may be operating based on datapreviously received. Initial delays added by the clock recovery register220 may be compensated for by relying on the clock 230 or by discardingthe data stream within the data path register 215 that precedes the dataassociated with the recovered clock signal.

The delay incrementer 225 may add a delay to the clock signal beforedisciplining the clock. In one embodiment, the delay or delta value maycorrespond to synchronization with a clock tick. For example, processingby the clock recovery register 220 may require 25.6 milliseconds, butclock ticks are tracked to milliseconds, as a result, the delayincrementer may add 0.4 milliseconds to the clock signal beforesynchronizing the clock 230 or communicating the data stream from thetransmitter 235. In some embodiments, the delay incrementer 225 may notbe included at all or alternatively may be set to introduce no delay.

The recovered clock signal output by the clock recovery register 220 isutilized to discipline the clock 230 may be performed without affectingdelay requirements for buffering data. For example, ticks of the clock230 or other time intervals of the clock 230 may be synchronized withthe recovered clock signal received from the clock recovery register220.

The illustrative embodiments may allow the incoming signal to beduplicated for parallel processing to compensate for jitter and othersignal problems while simultaneously recovering a clock signal to updatethe clock for further communication of the original incoming signal. Asa result, smaller registers may be utilized and data may be moreefficiently played out for additional communications.

FIG. 3 is a flowchart of a process for recovering a clock in accordancewith an illustrative embodiment. The process of FIG. 3 may beimplemented by the CPE device or a clock recovery system of acommunications device. The process may begin by separating an incomingsignal into two separate signals (step 302).

Next, the clock recovery system manages jitter and data recovery (step303). During step 303, the clock recovery system may account for jitter,latency, out of order packets, and other conditions and factors that mayeffect the viability and reliability of the data stream passing throughthe clock recovery system. The clock recovery system may compensate forjitter, order, and data recovery utilizing a first signal generated orseparated in step 302. For example, a first register may be utilized tocompensate for jitter and order packets as necessary before playing outthe packets. For example, the waveforms, bits, bytes, packets, or framesmay be played out at a specified rate for processing by a CODEC for a T1or similar connection.

Simultaneously, the clock recovery system retrieves the clock signal(step 304). The clock signal may be recovered utilizing a second signalcreated or separated during step 302. In one embodiment, a second shiftregister may be utilized to buffer the packets in order to extract theoriginal clock signal. The functions and operations of steps 303 and 304may be performed in parallel utilizing multiple registers so that asingle larger register may not need to be utilized. The clock signal maybe retrieved in step 304 utilizing any number of methods and processesutilized for pseudowire as is commonly known in the art. The registermay be of any number of sizes and introduce various time delays in orderto most accurately recover a clock signal. As a result, the data streammay not be played out based on real time or direct recovery of the clocksignal from the data path register. A larger delay in the clock recoveryregister may allow the recovered clock signal to be retrieved moreaccurately.

Next, the clock recovery system adds an amount of delay to the recoveredclock signal (step 306). The amount of time added in step 306 may be adelta value that ensures that the processing time required by the clockrecovery system corresponds to ticks of a clock.

Next, the clock recovery system discards the second signal (step 308).The secondary signal and correspond bits and packets are discardedbecause two separate signals are generated in step 302. The secondsignal may only be utilized for clock recovery, and as a result, onceclock recovery is performed, the secondary signal may be discarded.

Next, the clock recovery system disciplines a clock (step 310). Theclock may be a secondary reference clock utilized by the CPE. Forexample, the clock may be a stratum 2 clock utilized by secondary nodesin a communications network. In one embodiment, the clock signalrecovered and modified in step 306 is utilized to discipline the ticksor timestamps of the clock. Any number of disciplining systems,standards, or schemas may be utilized. The accurate clock ensures thatcommunications occur according to a single standard or clock referencedespite propagation delays, processing times, and other factors orcommunications characteristics that may affect utilization of a singleclock signal in multiple devices or systems. In one embodiment, theclock recovery system may not include the clock of step 310 and as aresult the recovered clock signal may be utilized to playout orotherwise communicate the data stream to other entities.

Next, the clock recovery system sends communications utilizing the clock(step 312). The communications may be TDM communications that require aclock signal. In another embodiment, the data received by the clockrecovery system may be further processed utilizing the clock system.

The previous detailed description is of a small number of embodimentsfor implementing the invention and is not intended to be limiting inscope. The following claims set forth a number of the embodiments of theinvention disclosed with greater particularity.

What is claimed is:
 1. A method for separating clock recovery for apseudowire communication, the method comprising: receiving an incomingsignal for a pseudowire communication; separating the incoming signalinto a first signal and a second signal; ordering packets with the firstsignal in a first register; extracting a clock signal from the secondsignal in a second register to generate a modified clock signal;incurring a processing delay during generation of the modified clocksignal; rounding the processing delay to a tracked time increment forclock ticks; adding the rounded processing delay to the modified clocksignal; and communicating the first signal utilizing the modified clocksignal.
 2. The method according to claim 1, wherein the separating isreplicating.
 3. The method according to claim 1, wherein the separatingis duplicating.
 4. The method according to claim 1, wherein the incomingsignal is a packet stream received through a metro Ethernet network. 5.The method according to claim 1, further comprising: disciplining aclock utilizing the modified clock signal after adding the roundedprocessing delay to the modified clock signal.
 6. The method accordingto claim 5, wherein the disciplining is performed for ticks of areference clock encoded in the incoming signal.
 7. The method accordingto claim 6, wherein the disciplining is performed out of phase with thefirst signal in response to the delay.
 8. The method according to claim1, wherein the communicating comprises: playing out the first signalutilizing the modified clock signal.
 9. The method according to claim 1,wherein the first signal is received on a first network, and wherein thefirst signal is communicated on a second network.
 10. The methodaccording to claim 9, wherein the first network is operated by a firstcommunication service provider, and wherein the second network isoperated by a second communication service provider.
 11. A clockrecovery unit comprising: a processor for executing a set ofinstructions; and a memory for storing the set of instructions, whereinthe set of instructions are executable by the processor to: receive anincoming signal for a pseudowire communication; separate the incomingsignal into a first signal and a second signal; order packets with thefirst signal in a first register; extract a clock signal from the secondsignal in a second register to generate a modified clock signal; incur aprocessing delay during generation of the modified clock signal; roundthe processing delay to a tracked time increment for clock ticks; addthe rounded processing delay to the modified clock signal; andcommunicate the first signal utilizing the modified clock signal. 12.The clock recovery unit according to claim 11, wherein the instructionsexecutable to separate the incoming signal comprise instructionsexecutable to replicate the incoming signal.
 13. The clock recovery unitaccording to claim 11, wherein the instructions executable to separatethe incoming signal comprise instructions executable to duplicate theincoming signal.
 14. The clock recovery unit according to claim 11,wherein the incoming signal is a packet stream received through a metroEthernet network.
 15. The clock recovery unit according to claim 11,wherein the set of instructions further comprise: instructionsexecutable to discipline a clock utilizing the modified clock signalafter adding the rounded processing delay to the modified clock signal.16. The clock recovery unit according to claim 15, wherein theinstructions executable to discipline a clock comprise instructions todiscipline a clock for ticks of a reference clock encoded in theincoming signal.
 17. The clock recovery unit according to claim 16,wherein the instructions executable to discipline a clock compriseinstructions to discipline a clock out of phase with the first signal inresponse to the delay.
 18. The clock recovery unit according to claim11, wherein the instructions to communicate the first signal compriseinstructions executable to play out the first signal utilizing themodified clock signal.
 19. The clock recovery unit according to claim11, wherein the first signal is received on a first network, and whereinthe first signal is communicated on a second network.
 20. The clockrecovery unit according to claim 19, wherein the first network isoperated by a first communication service provider, and wherein thesecond network is operated by a second communication service provider.